In general, an injection molding machine comprises a mold or a die, an injection section for injecting a molten resin to the die, and a control section for controlling operation of the injection section. The control section controls a sequence of injection pressures and time durations for a sequence of steps from a filling step through a holding step with reference to various detection signals extracted from the injection section and set values.
Generally, control conditions at the control section widely differ dependent on shapes of molded products to be formed. Accordingly, it is the practice to individually determine set values supplied to the control section for each molded product. Particularly, setting of injection pressures and control of injection time durations at the control section must be precisely determined because they are significant elements which determine the quality and accuracy of the molded products.
Heretofore, in order to set injection pressures during each step of an injection process, a method is adopted which at first prepares a defined pattern of the injection pressures in correspondence with the molded product in careful consideration of a feature and a shape of a molded product as well as a characteristic of a resin and switches stepwise the injection pressures at predetermined time intervals according to the defined pattern.
It is to be noted here that the defined pattern of the injection pressures is determined by parameters such as a thickness of the molded product, a shape of the molded product, and a liquidity degree of a molten resin. For example, as the molded product becomes thicker and its shape becomes more complicated, a uniformity of the molded product can not be assured unless the defined pattern has more multiple stages, while sinks and burrs are susceptible to be formed. However, when the defined pattern has multiple stages, setting points are disadvantageously increased in number. In addition, there is another problem that much skill is required for accurate judgment of an optimum value to be set for each stage.
Moreover, when the defined pattern has multiple stages, overshoot occurs every time on transition between stages. Accordingly, discontinuous points appear in the injection pressure actually applied to the die. This results in a disadvantage of leakage of a resin already charged in the die. Although various methods are known for removing such discontinuous points, there remains a problem in practical use because a structure of the injection section or the control section becomes complicated with any of those methods.
On the other hand, in case when a molded product is of a simple shape or when a molten resin has a low liquidity, it is possible to assure a quality of the molded product without switching the multiple stages. However, various kinds of particular defined patterns must be prepared in correspondence with characteristics of various molded products. This results in insufficiency in general use. In addition, there is another disadvantage that a high technology is required to produce such defined patterns.
Taking the above-mentioned problems into consideration, it is an object of the present invention to provide an injection pressure setting method capable of readily preparing characteristics in correspondence with molded products.
It is another object of the present invention to provide an injection molding machine which can be generally used and which can readily control an injection pressure.